GLOSSARY 



475 



DICHIRUS - partial duplication of digits in hand 

 or foot, possibly inherited. A type of Poly- 

 dactyly. 



DICHOTOMY, DIFFERENTIAL - embryonic 

 segregation; capacity of embryonic cells 

 for self-differentiation becomes itself dif- 

 ferentiated (Lillie, 1929). 



DIFFERENTIATING CENTER - area responsible 

 for the localization and determination of 

 various regions of the embryo, resulting in 

 harmonious proportioning of parts. 



DIFFERENTIATION - acquisition of specialized 

 features which distinguish areas from each 

 other; progressive increase in complexity 

 and organization, visible and invisible; 

 elaboration of diversity through determina- 

 tion leading to histogenesis; production of 

 morphogenetic heterogeneity. Syn. , differ- 

 enzierung. 



DIFFERENTIATION, AXIAL - variations in den- 

 sity of chemical and often indefinable inclu- 

 sions in the direction of one diameter of the 

 egg, called the egg axis (see gradient). 



DIFFERENTIATION, CELLULAR - the process 

 which results in specialization of a cell as 

 measured by its distinctive, actual, and 

 potential functions (Bloom, 1937). 



DIFFERENTIATION CENTER - one of the true 

 organization centers in the developing insect 

 egg. Syn. , differenzierungszentrum (of 

 Lehmann, 1942). 



DIFFERENTIATION, CORPORATIVE - differ- 

 entiation resulting from the physiological 

 functioning of parts. 



DIFFERENTIATION, DEPENDENT - all differ- 

 entiation that is not self-differentiation; the 

 development of parts of the organism under 

 mutual influences, such influences being 

 activating, limiting, or inhibiting. Inability 

 of parts of the organism to develop inde- 

 pendently of other parts. Such a period in 

 ontogeny always precedes that of irreversible 

 determination. "Experimental embryology 

 is a study of the differentiations which are 

 dependent, causally effected." (Roux, 1912). 

 Syn. , correlative differentiation, Abhangige 

 Differenzierung, Differentiation provoquee. 



DIFFERENTIATION, FUNCTIONAL - differen- 

 tiation of tissues resulting from forces asso- 

 ciated with functions (stresses and strains) 

 which they are performing. 



DIFFERENTIATION, INDIVIDUATIVE - differen- 

 tiation due to the action of morphogenetic 

 fields rather than to physiological function- 

 ing of parts; opposed to corporative differ- 

 entiation. 



DIFFERENTIATION POTENCY - the total reper- 

 toire of differentiations, cytological and his- 

 tological, available to a given cell. Wider 

 significance than prospective fate. 



DIFFERENTIATION, REGIONAL - refers to 



fact that different parts of the organizer will 

 induce different end organ formation; also 

 refers to organ districts in a (limb) field. 



DIFFERENTIATION, SELF - the perseverance 

 in a definite course of development of a 

 part of an embryo, regardless of its altered 

 surroundings (Roux, 1912). Syn., differen- 

 tiation spontane'e. 



DIKENETIC - dicentric, having. two kinetochores. 



DIMEGALY - possessing spermatozoa of two 

 sizes. 



DIPHYGENIC - having two types of development. 



DIPLICHROMOSOME - two identical chromo- 

 somes, held together at the kinetochore and 

 originated by doubling of chromosomes with- 

 out separation of daughter chromosomes. 



DIPLOID - normal number of chromosomes, 



double the gametic or haploid; complete set 

 of paired chromosomes as in the fertilized 

 egg or somatic cell. 



DIPYGUS - (See duplicatus inferior. ) 



DISSOGENY - having two sexually mature peri- 

 ods, one as a larva and one as an adult. 



DISTRICT - a portion of a morphogenetic field 

 with certain specific determinations. Syn. , 

 territoire. 



DIVERGENCE. VENTRAL - divergence of ma- 

 terial from the mid-ventral line, compensa- 

 tory to the process of dorsal convergence in 

 gastrulation (Vogt). 



DIVISION HIERARCHY - "four dimensional 

 array of cells of which one and only one 

 member (the zygote) is before all other 

 members in time, and is the only one to 

 which every other term stands in a relation 

 which is some power of D (i. e. , the relation 

 is Dpo). " (Bertelanffy h Woodger, 1933.) 



DOMINANCE - in embryology this term refers 

 to parts of a system which have greater 

 growth momentum and also which gather 

 strength from the rest, such as the dorsal 

 blastoporal lip. 



DONNAN EQUILIBRIUM - distribution of ions on 

 two sides of a semi -permeable membrane 

 with diffusion until concentration of diffusible 

 ions on the two sides of the membrane is the 

 same, involving ionic rather than molecular 

 balcLnce. 



DOPA - 3:4:dioxyphenylalanin, an intermediate 

 oxidation product of tyrosine and one that 

 appears as a precursor of melanin pigment 

 in mammals (Black, 1917). 



DORSO-VENTRAL - orientation of a graft or 



transplant so that the original dorsal-ventral 

 axis is inverted in relation to the host field. 



D-QUADRANT - one of the four early blasto- 

 meres of the annelid embryo which has the 

 prospective function of giving rise chiefly 

 to mesoderm. 



